Metabolic changes associated with sustained 48-hr shivering thermogenesis were studied in piglets maintained at 34 (thermoneutrality) or 25 degrees C (cold) between 6 and 54 hr of life. Despite their high shivering activity and elevated heat production, cold-exposed piglets exhibited a slightly lower rectal temperature than thermoneutral animals (-1.1 degrees C; P < 0.01) at the end of the treatment. The enhancement of heat production and shivering activity were associated with a decrease in muscle glycogen (-47%; P < 0.05) and total lipid content (-23%; P < 0.05), a reduction of blood lactate levels (P < 0.05) and an enhancement of muscle cytochrome oxidase activity (+20%; P < 0.05), which suggests that muscle oxidative potential was increased by cold exposure. Potential for capturing lipids (lipoprotein lipase activity) was also higher in the red rhomboideus muscle (+71%; P < 0.01) and lower in adipose tissue (-58%; P < 0.01) of the cold-exposed piglets. Measurements performed at the mitochondrial level show no changes in rhomboideus muscle, but respiratory capacities (state IV and FCCP-stimulated respiration) and intermyofibrillar mitochondria oxidative and phosphorylative (creatinine kinase activity) capacities were enhanced in longissimus dorsi muscle (P < 0.05). These changes may contribute to provide muscles with nonlimiting amount of readily oxidable substrates and ATP necessary for shivering thermogenesis. A rise in plasma norepinephrine levels was also observed during the second day of cold exposure (P < 0.05).